A harsh environment usually involves one or more of the following: extreme temperature, high pressure, high shock, high radiation, and chemical attack. High temperature electronics and sensors that are able to operate between 300 °C to 600 °C have drawn a lot of attention due to their wide applications in harsh environment such as in oil/gas exploration, geothermal development, industrial manufacturing processes, and space exploration. Silicon carbide (SiC) has become a great candidate as an electrical material for these harsh environment applications because of its wide bandgap, its high temperature operation ability, its excellent thermal and chemical stability, and its high breakdown electric field strength. In this work, the stable performance of 4H-SiC p-n diodes from room temperature to 600 °C has been demonstrated. Simulation, fabrication and characterization of the 4H-SiC p-n diodes are performed. The simulated 4H-SiC p-n diode shows that its turn-on voltage changes from 2.7 V to 1.45 V when temperature increases from 17 oC to 600 oC. The fabricated 4H-SiC p-n diode has a turn-on voltage from 2.6 V to 1.3 V with temperature rising from 17 oC to 600 oC, which are in good agreement with the simulation results. The demonstration of the stable operation of the 4H-SiC p-n diodes at high temperature up to 600 oC shows great potentials for 4H-SiC devices and circuits working in harsh environment electronic and sensing applications.